One possible enemy could be a sorcerer who is jealous of Percy's shapeshifting ability. The sorcerer might try to steal the power or use it for his own evil purposes. Another enemy could be an ancient mythical beast that sees Percy's shapeshifting as a threat to its territory. For example, a giant sea serpent that doesn't want Percy changing into different sea creatures and disturbing the balance of the ocean.
In this fanfiction, the gods themselves could be Percy's enemies. Maybe they see his shapeshifting ability as a disruption to the natural order. Zeus could be worried that Percy might use his shapeshifting to bypass his thunderbolts or other powers. Hades might think Percy could use it to infiltrate the Underworld too easily. So, they send various challenges and monsters his way to test or stop him.
There could be a group of dark demigods who view Percy's shapeshifting as an unfair advantage. They might team up to try and defeat him and take away his power. They could be led by a particularly devious demigod who has a vendetta against Percy for some past event. Or maybe a powerful deity from another pantheon who doesn't like the idea of a demigod having such a unique ability and decides to send minions to hunt Percy down.
Some of the more traditional and rule - following demigod groups at Camp Half - Blood could be against him. They believe in following the established ways and Percy's rogue behavior goes against that. They might see him as a troublemaker and try to bring him back to the 'right' path.
As for enemies, Percy could encounter the Titans who were once banished there. They might see Percy as an intruder and a threat, and try to eliminate him. Also, the monstrous creatures that lurk in Tartarus, like the Empousai or the Chimera, could be his foes.
If Percy joins the Titans, his allies could be some of the lesser - known Titans who are tired of the constant fighting. They might see Percy as a way to bring about change. Enemies? Well, the Titans who are fully committed to overthrowing the Olympians and establishing their own tyrannical rule would be against him. Also, Poseidon, his father, might be conflicted about Percy's choice and could end up being an enemy if Percy doesn't change his course.
The Titans could be potential enemies. Even though they've been defeated before, some of them might be resentful and see Percy as the Chaos Champion as a new threat to their possible plans of a comeback. Kronos, for example, might have some of his essence lingering and try to possess other powerful beings to go against Percy.
The new enemies could be the descendants of an ancient and forgotten race of sea - dwelling creatures who are jealous of Percy's growing power. They might see him as a threat to their long - lost dominion over the oceans. Another possibility is a group of rogue demigods who have been corrupted by dark magic and want to steal Percy's new powers for themselves. There could also be a powerful sorcerer from a far - off land who has heard of Percy's powers and wants to use them to break a curse that has been placed on him.
The children of Ares might see Percy as a threat. Since Cupid represents love and Ares represents war, their natures are quite opposite. The Ares kids could be jealous of Percy's powers or simply not trust him because of his different lineage.
The traditional enemies of the demigods, like the Titans or the Giants, could still be his enemies. They might see him as an abomination because of his father being Charon. Also, some of the more puritanical demigods who follow the Olympian gods strictly might oppose him.
The Titans could still be their enemies. Even in the realm of Chaos, the Titans might see them as a threat to their potential return to power. Kronos, if he somehow still has a presence, would likely oppose them. He would not want these three, especially Percy who has defeated him before, to gain more power from Chaos.
The Titans could still be a threat. Even though they were mostly defeated, there could be some remnants or new Titans emerging. For example, a Titan who was in hiding and now sees Percy as a threat to his plans of taking over again.
In other cases, it might be a result of a curse or a spell gone wrong. His enemies could be trying to harm him with a dark spell, but instead, it backfires and he ends up absorbing their power. For example, if they were trying to drain his own power but the spell reversed on them.